Double dipping — used in the classic context of a merchant running an affiliate program on two (or more) affiliate networks — is an instance when an affiliate receives their commission twice (or, figuratively speaking, dips his/her cracker into the commission sauce twice): on each of the two (or more) networks. This, naturally, poses a problem for the merchant, as instead of paying the agreed-upon commission rate they are paying twice as much.
In my experience such a problem almost always occurs due to the merchant’s fault. Most frequently, double dipping happens in situations when merchants place both tracking codes (I’ll continue using two networks in my example, even though the number is can be greater than two) into the code of the same “thank you” page. Consequently, if one affiliate is actively promoting the merchant through one affiliate network’s links, while the other one is doing the same through the other network, when a sale happens, both would get the commission. Similarly, if one affiliate is promoting the merchant aggressively through both affiliate networks, it is not unusual for some of the orders they refer to be “dipped” twice as well.
How can a merchant ensure this doesn’t happen? You want to either (i) display a dynamically refreshing tracking code, corresponding to network’s link initially clicked (keep in mind that some networks prohibit this; so read on), or (ii) have two different “thank you” pages, each of which would have only one network’s tracking pixel, and display the one which corresponds to the affiliate network’s link clicked last (i.e. if it were a ShareASale affiliate who referred the client, you would display the “thank you” page with the ShareASale’s tracking pixel, while if it were a CommissionJunction affiliate — display the page that has the CJ’s tacking in the code).